Recently, CMOS devices are fabricated, in some cases, by performing ion implantation through a mask of KrF resist film to form p- and n-wells. As the size of resist patterns is reduced, more attention is paid to ArF resist films. For further miniaturization, ArF immersion lithography is proposed. The substrate surface must be bare in spaces of a resist film before ion implantation can be carried out. This is because if a bottom antireflection coating (BARC) layer is present below the resist film, ions are trapped by the BARC layer. However, if the photoresist film is patterned in the absence of BARC layer, standing waves are generated due to substrate reflection, resulting in substantial corrugations in the sidewall of resist pattern after development. For the purpose of smoothing out a corrugated profile due to standing waves, it is believed effective to use a photoacid generator (PAG) capable of generating a low molecular weight acid amenable to more acid diffusion or to apply high-temperature PEB because acid diffusion is enhanced by either means. In the size range of 200 to 300 nm where the resist film for ion implantation is resolved by KrF lithography, resolution is not degraded by the enhancement of acid diffusion. In the size range of less than 200 nm where the resist film for ion implantation is resolved by ArF lithography, however, the enhancement of acid diffusion is undesirable because resolution is degraded or proximity bias is enlarged by acid diffusion.
It was contemplated that the substrate surface to be ion implanted is made bare by placing a BARC film as a layer beneath a resist film, developing the resist film to form a resist pattern, and dry etching the BARC film with the resist pattern made mask. In this approach, soft dry etching is employed so that the substrate may not be altered, for the reason that if the substrate is oxidized to form an oxide layer, ions can be trapped by this portion. Specifically, dry etching with hydrogen gas is used because the substrate can be oxidized by dry etching with oxygen gas. Then a BARC having a high dry etching rate with hydrogen gas is required.
A dyed resist material is the most traditional technique, which is based on the concept that a photoresist film itself is made absorptive for preventing generation of standing waves, and has been investigated since the age of i or g-line novolak resist materials. As the absorptive component used in ArF lithography, a study was made on a base polymer having benzene ring introduced therein or an additive having benzene ring. However, the absorptive component is insufficient to completely prevent standing waves. Increased absorption is effective for reducing standing waves, but gives rise to the problem that the resist pattern becomes tapered, i.e., of trapezoidal profile in cross section.
It was also contemplated to provide a top antireflection coating (TARC) film as a layer on the resist film. The TARC is effective for reducing standing waves, but not for preventing halation due to irregularities on the substrate. Ideally the refractive index of TARC is equal to the square root of the refractive index of the photoresist film. However, since the methacrylate used in the ArF resist film has a relatively low refractive index of 1.7 at wavelength 193 nm, there are available no materials having a low refractive index equal to its square root, 1.30.
Then a study was made on BARC which is dissolved in developer (see Proc. SPIE Vol. 5039, p 129 (2003)). At the initial, the study was directed to BARC which is anisotropically dissolved in developer. This approach was difficult in size control in that with the excess progress of dissolution, the resist pattern is undercut, and with short dissolution, residuals are left in spaces. Next the study was made on photosensitive BARC. In order that a film function as BARC, it must have an antireflective effect, remain insoluble in a photoresist solution which is coated thereon, and avoid intermixing with the photoresist film. If the BARC is crosslinked during post-application bake of BARC solution, it is possible to prevent the BARC from dissolution in the photoresist solution and intermixing therewith.
As the crosslinking mechanism during post-application bake, JP-A H06-230574 discloses to use vinyl ethers as the crosslinker. Specifically, a vinyl ether crosslinker is blended with hydroxystyrene, whereby crosslinking takes place during prebake after coating, forming a resist film which is insoluble in alkaline developer. Thermal reaction between vinyl ether group and phenol group creates an acetal group. The PAG generates an acid upon exposure. Then the acetal group is deprotected with the aid of acid, water and heat. The film functions as a positive resist film in that the exposed region is alkali soluble. This mechanism is applicable to dissolvable bottom antireflection coating (DBARC) as described in WO 2005/111724 and JP-A 2008-501985.
A substrate to be ion implanted has surface irregularities (or raised and recessed portions). The BARC becomes thicker on a recessed portion of the substrate. When DBARC is applied to a flat substrate, in the exposed region, the BARC film is dissolved in alkaline developer simultaneously with the photoresist film. When DBARC is applied to a stepped substrate, there arises a problem that the DBARC film on the recessed portion is not dissolved. Since DBARC has strong absorption, light does not reach the bottom as the DBARC film becomes thicker, so that the amount of acid generated by PAG in the DBARC is reduced. Particularly the thicker portion of DBARC film above the step is less sensitive to light in its proximity to the substrate and thus less dissolvable.
It is also contemplated to apply the trilayer process to ion implantation. In this case, a bottom layer of hydrocarbon is coated on a substrate and crosslinked during bake, a silicon-containing intermediate layer is coated thereon and crosslinked during bake, and a photoresist material is coated thereon. A resist pattern is formed via exposure and development. With the resist pattern made mask, the silicon-containing intermediate layer is dry etched with fluorocarbon gas. With the silicon-containing intermediate layer made mask, the bottom layer is processed by dry etching. With the bottom layer made mask, ions are implanted into the substrate. Although the dry etching for processing the bottom layer typically uses oxygen gas, dry etching with hydrogen gas capable of avoiding oxide formation is preferred for the reason that the substrate surface, if oxidized, becomes an ion stop during ion implantation, as discussed above. The trilayer process can prevent reflection off the substrate completely, so that the resist pattern on its sidewall may not be provided with any corrugations due to standing waves. Where a silsesquioxane-based SOG film is used as the silicon-containing intermediate layer, the SOG film having a high silicon content exhibits a high etching rate during dry etching of the silicon-containing intermediate layer with the resist pattern made mask and a slow etching rate during etching of the bottom layer, that is, exerting an excellent hard mask function, but suffers from the problem that it is not amenable to solution stripping after ion implantation. While the SOG intermediate layer is typically removed with hydrofluoric acid, the use of hydrofluoric acid causes significant damage to the substrate which is a silicon oxide film.
Substrate cleaning solutions which are commonly used in the art include an aqueous solution of ammonia and aqueous hydrogen peroxide (known as SC1), an aqueous solution of hydrochloric acid and aqueous hydrogen peroxide (known as SC2), and an aqueous solution of sulfuric acid and aqueous hydrogen peroxide (known as SPM). Most often, SC1 is used for cleaning of organic matter and metal oxide film, SC2 for removal of metal contamination, and SPM for removal of organic film. SOG film cannot be stripped with these cleaners. The SOG film is removed by dry etching with CF base gas and cleaning with dilute hydrofluoric acid or a combination of dilute hydrofluoric acid and SPM, and the carbon underlayer film is removed by dry etching with oxygen or hydrogen gas or cleaning with SPM. When oxygen gas etching or SPM solution stripping is applied to the stripping of the underlayer film on a substrate which is a Si substrate, the substrate surface is oxidized into SiO2. Once the surface of Si substrate is converted to SiO2, the electrical conductivity is reduced to such an extent that the semiconductor may not perform. In contrast, the hydrogen gas etching does not oxidize the substrate, but has a slow etching rate, failing to remove phosphorus and arsenic present in the underlayer film after ion implantation. There is a need for an underlayer film which is solution strippable so that any concern about oxidation of the substrate surface is eliminated.